Polyolefin microporous membranes are used in various applications such as battery separators, separation membranes for electrolytic capacitors, various filters, moisture-permeable waterproof clothing, reverse osmosis filtration membranes, ultrafiltration membranes, and microfiltration membranes. When a polyolefin microporous membrane is used as a battery separator, particularly as a lithium ion battery separator, the performance of the membrane is highly responsible for battery properties, battery productivity, and battery safety. Therefore, the membrane requires excellent permeability, mechanical properties, heat shrinkage resistance, shutdown property, meltdown property, and the like. For example, if the membrane has low mechanical strength, when it is used as a battery separator, the voltage of the battery can be reduced by a short circuit between electrodes.
To improve the physical properties of polyolefin microporous membranes, studies have been made to improve material composition, stretching conditions, heat treatment conditions, and the like.